Conventionally, positioners for controlling the openings of valves have been, for example, positioners wherein the critical components have been configured as illustrated in FIG. 3. In this figure, 200 (200A) is a positioner, and 300 is a valve, where the valve 300 is provided with a position sensor 10 for detecting a position that indicates the degree of opening of the valve.
The positioner 200A is provided with an electric current-SP converting portion 1, a control calculating portion 2, an electropneumatic converter 3, a pilot relay 4, and a valve opening calculating portion 5.
In the positioner 200A, the electric current-SP converting portion 1 converts into an opening setting signal SP an input signal that is sent, as a signal of between 4 and 20 mA, from a higher level device. The valve opening calculating portion 5 calculates the current valve opening of the valve 200A from the position that indicates the opening of the valve, detected by the position sensor 10, and outputs, as an actual opening signal PV, a signal depending on the calculated opening. The control calculating portion 2 calculates the difference between the opening setting signal SP from the electric current-SP converting portion 1 and the actual opening signal PV from the valve opening calculating portion 5, and outputs, as a control signal MV, an electric signal obtained through performing PID control calculations on the difference.
The electropneumatic converter 3 converts into an air pressure (a nozzle back pressure) Pn the control signal MV from the control calculating portion 2. The pilot relay 4 uses the pneumatic signal Pn from the electropneumatic converter 3 as an input air pressure and amplifies this input air pressure Pn to produce an output air pressure Po, and outputs it to the operating device 11 of the valve 300. Doing so causes the air of the air pressure Po to flow into a diaphragm chamber within the operating device 11, to adjust the opening of the valve portion 12.
In this positioner 200A, that which is subject to control by the control calculating portion 2 includes the electropneumatic converter 3 and the pilot relay 4, which apply the air flow rate and the air pressure to the valve 300. If the characteristics of the electropneumatic converter 3 and the pilot relay 4 were linear, and were well-behaved characteristics, then such a control method would be adequate; however, in practice both the electropneumatic converter and the pilot relay 4 have non-linear elements such as hysteresis and a dead band, and so control is complex. Given this, because in the positioner 200A the feedback information is only the actual opening signal PV of the valve 300, if a change does not appear in the actual opening signal PV, then there is no change in the control output (the control signal MV). Because of this, controllability is a problem. Given this, a positioner with improved controllability has been proposed in Japanese Patent 3595554 (“the JP '554”).
FIG. 4 is a diagram illustrating schematically the positioner shown in the JP '554. In this positioner 200 (200B), the differential value dPo/dt of the output air pressure Po that is outputted from the pilot relay 4, as a pressure differential value, is fed back to the control calculating portion 2 in addition to the actual opening signal PV that indicates the current degree of opening of the valve 300. The output air pressure Po changes more rapidly than the actual opening signal PV. Because of this, feeding back the magnitude of change of the output air pressure Po makes it possible to estimate, in advance, the change that will occur in the actual opening signal PV. This enables superior control when compared to that of feeding back only the opening information.
However, in the positioner 200B disclosed in the JP '554, even though the differential information for the output air pressure Po (the control pressure for the diaphragm portion of the operating device 11) of the pilot relay 4, wherein the response is faster than that of the actual opening of the valve 300, is fed back, when there is a noise, and the like, in the differential information, the output becomes extremely large. Because of this, there is a problem in that the control tends to become unstable.
The present invention was created in order to solve the problem as set forth above, and an aspect thereof is to provide a positioner that improves substantially responsiveness without a loss of settling performance in control.